+1 here. I would use this a lot of theme options when it is related to transparent overlays. The plugin @herrfischer pointed out does do the trick but it is a bit clunky from a UX perspective. Personally, I would rather just be able to enter a hexidecimal the same as the color picker already does and then tick a radio button for rgb and alpha input.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users